It is known design practice to complement the braking action of friction wheel brakes of an automotive vehicle by using eddy current retarders. These convert the kinetic energy of the vehicle into electrical energy in the form of eddy currents. The electrical energy is dissipated in the form of heat.
Such eddy current brakes include a stator comprising either a permanent magnet or an electromagnet to create a magnetic flux field. The stator is stationary and a rotor is connected drivably to a torque delivery element, such as the drive shaft for automotive vehicle traction wheels.
In the case of an electromagnetic stator, the stator poles have electromagnetic windings that are excited with a voltage created by an onboard electrical power source, such as a vehicle battery alternator system.
An example of an eddy current brake with a permanent magnet stator has been described by H. Sakamoto et al in a publication entitled "Design of Permanent Magnet Type Compact ECB Retarder", SAE Technical Paper 973228, dated Nov. 17-19, 1997. An electromagnetic eddy current retarder is described by J. Bigeon et al in a paper entitled "Analysis of an Electromagnetic Brake", published in a journal entitled Electric Machines And Power Systems, 10:285-297, 1985.
Permanent magnet eddy current brakes have stator poles that are solid and, of necessity, are formed of magnetic materials with rare earth ingredients such as Neodymium Ferrous Boron (NdFeB). Such materials are both costly and difficult to maintain and handle due to their corrosion characteristics. Although an electromagnetic eddy current brake is less costly because of the lack of expensive materials, such as Neodymium Ferrous Boron, they too typically are of substantial weight because of the solid design of the stator.
Both permanent magnet eddy current brakes and electromagnetic eddy current brakes comprise a drive shaft and a rotor drum that rotate together in a magnetic field established by the stator windings. The braking force generated at the rotor is created by interaction of rotor eddy currents and the stator magnetic field. The eddy currents are formed principally at or near the outer surface of the rotor drum, depending on the speed of rotor rotation.
The kinetic energy involved in this braking action generates heat. Cooling fins usually are added to the outer surface of the rotor drum to dissipate the heat produced by the eddy currents.
Aside from the weight and cost disadvantages of presently known eddy current brakes, the design of known brakes requires complex assembly procedures because of the number of parts involved in the assembly. These parts include seals and steel closure plates that separate the eddy current brake from harsh environments to prevent rust and corrosion.